The present invention relates to a contents data transmission/reception system in which contents data, such as audio data and video data, is transmitted and received between plural apparatuses, a contents data transmitter, a contents data receiver, and a contents data transmission/reception method.
There is a digital versatile disc (DVD) as a recording medium which can record massive data. The DVD includes a DVD-VIDEO in which video data and audio data are recorded, and a DVD-AUDIO in which high quality audio data is recorded. An optical disc reproducing apparatus for reproducing these DVDs reproduces digital data recorded in the DVD, and can analog-output audio data as an analog signal or digital-output it as two-channel digital data.
There is a case where audio data of plural channels (multi-channel) such as a left(L)-channel, a right(R)-channel, a surround left(SL)-channel, a surround right (SR)-channel, and a center(C)-channel are recorded in the DVD. In the case where the multi-channel audio data recorded in the DVD is digital-outputted, the optical disc reproducing apparatus down-mixes the multi-channel audio data into 2-ch digital data, and outputs the down-mixed data as digital data in compliance with a data format of IEC (International Electrotechnical Commission) 958.
There is a case where the sound quality of the audio data analog-outputted from the optical disc reproducing apparatus is degraded since noise is superposed during signal transmission. As compared with analog output audio data, the sound quality of the audio data digital-outputted from the optical disc reproducing apparatus is not degraded by the superposition of noise during signal transmission.
When the audio data is digital-outputted, the optical disc reproduction apparatus converts the multi-channel audio data recorded in the DVD into the 2-channel audio data and outputs it. An amplifier (for example, AV (Audio Visual) amplifier) for amplifying the audio data outputted from the optical disc reproducing apparatus and outputting it to a speaker or the like receives the 2-channel audio data digital-outputted from the optical disc reproducing apparatus, converts it into multi-channel audio data and outputs it.
In the amplifier, the processing of converting the input 2-channel audio data into the multi-channel audio data is performed by conversion means mounted in the amplifier. In this arithmetic processing, since parameters used for its processing method and calculation vary according to respective manufacturers of the conversion means (for example, DSP (Digital Signal Processor)), the inputted 2-channel audio data can not be faithfully converted into the audio data of the original multi-channel recorded in the DVD. In other words, the 2-channel audio data is converted into multi-channel audio data, so that the audio data the sound quality of which is more degraded than the multi-channel audio data recorded in the optical disc is outputted from the amplifier.
Thus, there has been raised a demand for faithfully reproducing high sound quality audio data recorded in an optical disc in such a manner that the foregoing conversion processing is not performed to the multi-channel digital data, the audio data reproduced by the optical disc reproducing apparatus is digital-outputted as the multi-channel audio data, and the amplifier as the receiving side of the digital data converts the multi-channel digital audio data into an analog audio signal and outputs it.
However, in the case where the multi-channel digital audio data is outputted from the optical disc reproducing apparatus, it becomes possible to copy the digital audio data on a recording medium. In order to prevent the illegal copy of the digital audio data, it is conceivable to use a technique to transmit and receive encrypted contents data between two apparatuses, like a data transmission system as disclosed in the Laid-Open Japanese Patent application publication No. 11-289323 (patent publication 1) or a contents distribution system as disclosed in the Laid-Open Japanese Patent application publication No. 2000-242604 (patent publication 2)
In the data transmission system disclosed in the patent publication 1, a client computer generates an encryption key and a decryption key, sends the encryption key and an image data transfer request to a server computer, the server computer encrypts the image data by the encryption key, transmits the encrypted image data to the client computer, and the client computer uses the held decryption key to decrypt the encrypted image data.
In the contents distribution system disclosed in the patent publication 2, when a user ID, an authentication ID and a download request for contents data are transmitted from a client to a server, in the case where the user ID and the contents data are registered in a table of the server, and the authentication ID held by the server is identical to the authentication ID transmitted from the client, the server generates a decryption key for decrypting the encrypted contents, and transmits the encrypted contents data and the decryption key to the client. The client uses the decryption key transmitted from the server to decrypt the encrypted contents data, and reproduces the contents data.
In the data transmission system disclosed in the patent publication 1 or the contents distribution system disclosed in the patent publication 2, these systems encrypt the contents data by using one key information, and transmit the encrypted contents data and the encryption breaking information (key information) for decrypting the encryption of the encrypted contents data by using a single signal line for connecting the apparatuses.
In the system as described above, since the contents data is encrypted only by using one key information, and encryption decrypting information (key information) is transmitted as one of information through the single signal line from the contents data transmitter to the contents data receiver, there is a problem that the original high quality contents data can be obtained by extracting the encryption breaking information from the data transmitted between the two apparatuses and decrypting the encrypted contents data by using the encryption breaking information.